Laser diode devices have been widely used as light sources that emit signal light for optical communications. Laser diode devices are generally driven by laser drive circuits that also supply driving currents to the laser diode devices. Such laser drive circuits are roughly classified into two types: DC-coupled laser drive circuits and AC-coupled laser drive circuits.
A typical structure of a conventional DC-coupled laser drive circuit is illustrated in (a) of FIG. 7. (a) of FIG. 7 is a block diagram schematically illustrating a structure of a conventional DC-coupled laser drive circuit 100.
The DC-coupled laser drive circuit 100 is a circuit for supplying, to a laser diode device LD, a driving current ILD which varies depending on a voltage (electrical potential difference) of an input signal (a voltage signal that has been modulated by a data signal). As illustrated in (a) of FIG. 7, the DC-coupled laser drive circuit 100 includes an input buffer 110, a pre-driver 120, a main driver 130, a voltage drop section 140, and a constant electric current source 150.
The input buffer 110 is a circuit for matching between (i) input impedance (load impedance) in the DC-coupled laser drive circuit 100 and (ii) characteristic impedance of a transmission line. The pre-driver 120 is a circuit for adjusting an amplitude of an input signal supplied via the input buffer 110. The main driver 130 is a circuit for converting, into an electric current signal, the input signal whose amplitude has been adjusted by the pre-driver 120. The voltage drop section 140 is a voltage drop device such as a fixed resistance or a diode, and is provided between power source voltage VCC and the pre-driver 120.
The main driver 130 is connected to the laser diode device LD and the constant electric current source 150. The laser diode device LD receives the driving current ILD which is equal to a subtraction of Imod from Idc (ILD=Idc−Imod), where (i) the Imod indicates an inflow electric current flowing into the main driver 130 and (ii) the Idc indicates an outflow electric current flowing out of the constant electric current source 150. In a case where a voltage of the input signal is low, an electric current of the inflow electric current Imod becomes high, and consequently an electric current of the driving current ILD becomes low. In contrast, in a case where a voltage of the input signal is high, an electric current of the inflow electric current Imod becomes low, and consequently an electric current of the driving current ILD becomes high.
Note that the outflow electric current Idc flowing out of the constant electric current source 150 is controlled such that an electric current of bias electric current Ibias is greater than that of threshold laser electric current Ith. Note that the bias electric current Ibias herein refers to an electric current of the driving current ILD while a low voltage is being supplied to the laser diode device LD as the input signal.
A typical structure of a conventional AC-coupled laser drive circuit is illustrated in (b) of FIG. 7. (b) of FIG. 7 is a block diagram schematically illustrating a structure of a conventional AC-coupled laser drive circuit 200.
As is the DC-coupled laser drive circuit 100, the AC-coupled laser drive circuit 200 is a circuit for supplying, to a laser diode device LD, driving current ILD which varies depending on a voltage of an input signal. As illustrated in (b) of FIG. 7, the AC-coupled laser drive circuit 200 includes an input buffer 210, a pre-driver 220, a main driver 230, a voltage drop section 240, and a constant electric current source 250.
Functions of the input buffer 210, the pre-driver 220, the main driver 230, the voltage drop section 240, and the constant electric current source 250 illustrated in (b) of FIG. 7, are similar to those of the input buffer 110, the pre-driver 120, the main driver 130, the voltage drop section 140, and the constant electric current source 150 illustrated in (a) of FIG. 7, respectively.
The AC-coupled laser drive circuit 200 differs from the DC-coupled laser drive circuit 100 in that a capacitor 260 is interposed between the main driver 230 and the laser diode device LD. This prevents, even in a case where the main driver 230 of the AC-coupled laser drive circuit 200 outputs output voltage Vout involving a DC component, the DC component from being supplied to the laser diode device.
Compared with the AC-coupled laser drive circuit 200, the DC-coupled laser drive circuit 100 has the following advantages:
1) With the AC-coupled laser drive circuit 200, it is necessary to use, as a capacitor 260, a large capacitor having a capacitance of approximately 0.1 μF. This hinders the AC-coupled laser drive circuit 200 from being space-saving. On the other hand, the DC-coupled laser drive circuit 100 does not require such a large capacitor, and can therefore save space without difficulty.
2) According to the AC-coupled laser drive circuit 200, the capacitor 260 blocks low-frequency components of the driving current ILD. This hinders the AC-coupled laser drive circuit 200 from becoming broadband compatible. On the other hand, the DC-coupled laser drive circuit 100 does not require such a capacitor, and can therefore become broadband compatible without difficulty.
3) With the AC-coupled laser drive circuit 200, it is necessary to suppress output impedance of the main driver 230 low, and therefore loss of the driver electric current ILD can easily become significant. This hinders the AC-coupled laser drive circuit 200 from becoming power saving. With the DC-coupled laser drive circuit 100, on the other hand, there is no necessity to suppress output impedance of the main driver 130 low, and it is therefore easy to save electric power.
For example, a laser drive circuit to be mounted on an AOC (Active Optical Cable), which laser drive circuit is required to be space-saving, is preferably a DC-coupled laser drive circuit. As another example, a laser drive circuit for generating a burst signal in a network such as a PON (Passive Optical Network) must be a DC-coupled laser drive circuit having no concern of blocking low-frequency components.
Patent Literature 1 is mentioned below as an example of literatures disclosing DC-coupled laser drive circuits.